worst

adj.

Old English wyrresta, from Proto-Germanic *wers-ista- (cf. Old Saxon wirsista, Old Norse verstr, Old Frisian wersta, Old High German wirsisto), superlative of PIE *wers- "to confuse, mix up" (see worse). Phrase in the worst way (1839) is from American English sense of "most severely."

at worst

1.

Also,at the worst.

2.

In the least favorable circumstance; under the most difficult conditions. For example, Convicted of taking a bribe, the official believed that at worst he would be sentenced to a few months in prison.
[ 1500s
]

3.

In the least favorable view or supposition, as in No harm done; at the worst I'll copy the tax return again. Chaucer used this sense in Troilus and Cressida: “For at the worst, it may yet short our way.”
[ Late 1300s
]